Comparative study of the degradation rate of new and regenerated mineral oils following electrical stress

The objective of this contribution was to study the behaviour of new and regenerated insulating oil used in power transformers under the influence of an electric stress. To estimate the degradation rate of the dielectric fluids, one thousand (1000) successive breakdowns were generated according to the IEC 60156 standard. The parameters such as dissipation factor (Tan õ), resistivity, total acid number (TAN) and oil water Content were measured and examined following IEC/ISO standards. Good correlations have been obtained between TAN/resistivity and Tan õ which might provide a “picture” of the fluid condition. The dissolved oxidation products for the two dielectric fluids (after the application of electric breakdowns), was evaluated by infrared spectroscopy (FT-IR). The results obtained indicate that the degradation of the parameters is significant and confirms the influence of an alternative electric field (AC) on the new and regenerated oils. It was also suspected that inhibitors and antioxidants were removed from the oil after regeneration. Their concentration should therefore be monitored and replenished when necessary

Transformer oil reclamation by combining several strategies enhanced by the use of four adsorbents

From an environmental perspective, petroleum-based aged oils removed from power transformers are source of several pollutants and therefore cannot be disposed of without due care. The degradation of oil in in-service transformers is due to various factors concurrent with the operation of the units over several years. The present study proposes a new strategy to rejuvenate used mineral oils by combining centrifugation, dehydration and sorption with four different adsorbents: activated carbon (ACH), silica gel (SG), magnesium oxide (MO) and activated bentonite (AB). The process of regeneration proposed in this study resulted in a level of restoration that saw the used oil take on the characteristics of new oil (colour, dissipation factor, resistivity, permittivity, acid number). The results also showed that the optimum form of the re-refined base oil can be attributed to a 10% (w/w) quaternary mixture of the adsorbents, itself comprised of 1% ACH, 6% SG, 1% MO and 2% AB. The anticipated benefits are reduced risk of dielectric breakdown blamed for over 75% of extra high-voltage (EHV) power transformer failures and extended transformer life expectancy by retarding the solid insulation aging processes

The effect of water content on the electrical properties of transformer oil

The condition monitoring of insulating oil used in high voltage apparatus is based on assessing its electrical, physical and chemical properties. During their service life, these apparatuses such as transformers are subjected to several stresses which reduce their insulating capacity. Water and oxygen are the most important factors that accelerate the aging and degradation of transformer oil. There is a strong relationship between the presence of water in oil and the degradation of its electrical properties. This paper presents an experimental investigation to quantify and correlate the change in water content in transformer oil and its effect on their electrical properties. For this purpose, three samples of oil are used: new uninhibited, regenerated and used oil. A physical treatment using a MAXEI station is applied to the three samples to reduce their water content. The treatment is an operation of filtration, degassing and dehydration which is a necessary step before injection of the oil in transformer and during maintenance operations. During the physical treatment, the electrical properties of the oil samples are measured. The breakdown voltage, the dielectric loss factor, the permittivity and the resistivity were measured in according to the standards IEC 60156 and IEC 60247. The results revealed that for the three oil samples; new, regenerated and used, the electrical properties are strongly correlated with the change in water content in oil. The breakdown voltage and resistivity increase considerably when the water content is reduced. If the water content is reduced under 5 ppm, the breakdown voltage remains practically constant. The dielectric loss factor decreases by the water content variation. By decreasing the water content in oil, the insulating properties of transformer oil are improved. The analysis of the results shows that a strong correlation exists between the breakdown voltage, resistivity and the presence of water in oil

Transformer oil reclamation by combining several strategies enhanced by the use of four adsorbents

From an environmental perspective, petroleum-based aged oils removed from power transformers are source of several pollutants and therefore cannot be disposed of without due care. The degradation of oil in in-service transformers is due to various factors concurrent with the operation of the units over several years. The present study proposes a new strategy to rejuvenate used mineral oils by combining centrifugation, dehydration and sorption with four different adsorbents: activated carbon (ACH), silica gel (SG), magnesium oxide (MO) and activated bentonite (AB). The process of regeneration proposed in this study resulted in a level of restoration that saw the used oil take on the characteristics of new oil (colour, dissipation factor, resistivity, permittivity, acid number). The results also showed that the optimum form of the re-refined base oil can be attributed to a 10% (w/w) quaternary mixture of the adsorbents, itself comprised of 1% ACH, 6% SG, 1% MO and 2% AB. The anticipated benefits are reduced risk of dielectric breakdown blamed for over 75% of extra high-voltage (EHV) power transformer failures and extended transformer life expectancy by retarding the solid insulation aging processes